1. Field of the Invention
The present invention relates to a mobile communications system. More specifically, it relates to a radio-channel connection controller capable of distributing loads to relieve overloaded states such as a shortage of radio-channels in a given cell or a particular cell or spot inside a mobile radio communication network comprising a plurality of radio base stations. The present invention also relates to a mobile communication network system comprising such a connection controller.
2. Description of the Related Art
In the case of cellular systems using a conventional TDMA (Time Division Multiple Access) method such as GSM (Global System for Mobile Communication) and PDC (Personal Digital Cellular System), it has been necessary to add radio-channels or to add new base stations (mobile stations, etc.) when an overloaded state arises.
A TDMA system has a fixed number of radio-channels connectable through which user data are divided and transmitted by time division. In this system, changes in radio parameters and transmitting power extends its service area only a little. Further, even when reassignment of a radio resource from a cell next to the overloaded cell is tried, such reassignment is basically impossible because of the restriction on the frequency assignment. Accordingly, it has been necessary to provide an exclusive base station such as a micro-cell in an overloaded area of a particular cell. Therefore, a new frequency is required and, where the overloaded state does not occur too often, the utilization rate of equipment is low, which is not economical.
On the contrary, in a cellular system using a CDMA (Code Division Multiple Access) method, user data are divided and transmitted by a code-multiplexing technique. Therefore, the number of channels connectable can be changed according to the communication environment. Further, there is no restriction on the frequency assignment, the same frequency or radio-channel can be used among neighboring cells, and a load can be distributed among the neighboring cells.
Conventional technologies for distributing the load and raising the utilization rate of equipment of radio base stations in the mobile communication network are disclosed in the following documents. [Patent Document 1] Japanese Unexamined Patent Publication No. 8-205227 “Radio Communication system” [Patent Document 2] Japanese Unexamined Patent Publication No. 2000-333257 “System for Setting Path between Mobile Station and Radio Base Station”
The patent document 1 discloses a radio communication system capable of ensuring higher efficient use of a modulator-demodulator. In the radio communications system, a plurality of radio units only sending and receiving calls of radio signals as base stations of a PHS system and a plurality of modulator-demodulators provided in a centralized control station are connected via a connection unit capable of switching the connection. The connecting unit switches the connection per every time period wherein the number of calls in a radio zone differs.
The patent document 2 discloses a technology wherein a repeater or a radio relay unit is provided between neighboring radio base stations. When a cell corresponding to one radio base station is overloaded, the load is distributed between the neighboring radio base stations by using the repeater. Thus, communication capacities of radio base stations receiving fewer calls can effectively be utilized.
However, according to the method of the patent document 1, it is necessary to provide the modulator-demodulator to be shared in the centralized control station in the network. Therefore, this method cannot be applied to load distribution in such a communication network as a conventional network configuration having no centralized control station and a whole set of base transceiver station is provided in a cell site (where a cell is located) corresponding to each cell.
In a case where an overloaded state takes place in a wider area (a group of a plurality of cells), the neighboring radio base stations also become overloaded and a proper load distribution cannot be performed only by distributing the load through the repeater among the neighboring radio base stations as in the patent document 2. Also, the load distribution cannot be performed when a spot of a given size becomes overloaded inside the cell. Further, even if there are resources of a non-neighboring radio base station available, it cannot be utilized. In General, there is required a restriction on the connection of calls when overloaded, lowering communication quality such as a call-connection rate.